Achievement of distinctively high durability at nanosized Pt catalysts supported on carbon black for fuel cell cathodes

Solution-plasma interaction for synthesizing highly active Pt-Ni alloy oxygen reduction nanocatalysts for PEMFCs

A highly active PtNi-alloy catalyst (p-PtNi/KB) is synthesized using solution-plasma interaction. The plasma effect results in a substantial increase of active sites on the support surface. This facilitates the alloying and functionalization of supports, enabling achievement of enhanced catalytic activity and effective resolution of cost-related challenges in fuel cells.

Alternative and facile production pathway towards obtaining high surface area PtCo/C intermetallic catalysts for improved PEM fuel cell performance

The design of catalysts with stable and finely dispersed platinum or platinum alloy nanoparticles on the carbon support is key in controlling the performance of proton exchange membrane (PEM) fuel cells. In the present work, an intermetallic PtCo/C catalyst is synthesized via double-passivation galvanic displacement. TEM and XRD confirm a significantly narrowed particle size distribution for the catalyst particles compared to commercial benchmark catalysts (Umicore PtCo/C). Only about 10% of the mass fraction of PtCo particles show a diameter larger than 8 nm, whereas this is up to or even more than 35% for the reference systems. This directly results in a considerable increase in electrochemically active surface area (96 m² g^-1 vs. >70 m² g^-1), which confirms the more efficient usage of precious catalyst metal in the novel catalyst. Single-cell tests validate this finding by improved PEM fuel cell performance. Reducing the cathode catalyst loading from 0.4 mg cm^-2 to 0.25 mg cm^-2 resulted in a power density drop at an application-relevant 0.7 V of only 4% for the novel catalyst, compared to the 10% and 20% for the commercial benchmarks reference catalysts.

Advances in Low Pt Loading Membrane Electrode Assembly for Proton Exchange Membrane Fuel Cells

Hydrogen has the potential to be one of the solutions that can address environmental pollution and greenhouse emissions from traditional fossil fuels. However, high costs hinder its large-scale commercialization, particularly for enabling devices such as proton exchange membrane fuel cells (PEMFCs). The precious metal Pt is indispensable in boosting the oxygen reduction reaction (ORR) in cathode electrocatalysts from the most crucial component, i.e., the membrane electrode assembly (MEA). MEAs account for a considerable amount of the entire cost of PEMFCs. To address these bottlenecks, researchers either increase Pt utilization efficiency or produce MEAs with enhanced performance but less Pt. Only a few reviews that explain the approaches are available. This review summarizes advances in designing nanocatalysts and optimizing the catalyst layer structure to achieve low-Pt loading MEAs. Different strategies and their corresponding effectiveness, e.g., performance in half-cells or MEA, are summarized and compared. Finally, future directions are discussed and proposed, aiming at affordable, highly active, and durable PEMFCs.

Enhanced Ageing Performance of Sulfonic Acid-Grafted Pt/C Catalysts

Chemical functionalization of carbon support for Pt catalysts is a promising way to enhance the performance of catalysts. In this study, Pt/C catalysts grafted with various amounts of phenylsulfonic acid groups were prepared under mild conditions. The influence of sulfonic acid groups on the physiochemical characteristics and electrochemical activities of the modified catalysts were studied using X-ray diffraction, X-ray photoelectron spectroscopy, a transmission electron microscope, and cyclic voltammetry (CV). The presence of the chemical groups enhanced the hydrogen adsorption onto/desorption off the Pt surface during the CV cycling. In contrast, the hydrogen peaks of the grafted catalysts increased after 500 CV cycles, especially for Pt (111) facets. The highest electrochemical surface area (ECSA) after the aging test was obtained for the catalyst with 18.0 wt.% graft, which was ca. 87.3% higher than that of the non-functionalized Pt catalyst. In the density functional theory (DFT) calculation, it was proven that SO3H adsorption on the crystalline was beneficial for Pt stability. The adsorption energy and bond distance of the adsorbed SO3H on Pt (110), (100), and (111) surfaces were calculated. All the stable configurations were obtained when O from S-O single bond or S was bound to the Pt surface, with the adsorption energy following the trend of (111)F > (100)H > (110)H. This result was consistent with the ECSA experiment, which explained the high electrochemical stability of the sulfonic acid groups-grafted Pt/C catalyst.

Stability and activity of platinum nanoparticles in the oxygen electroreduction reaction: is size or uniformity of primary importance?

Platinum-carbon catalysts are widely used in the manufacturing of proton-exchange membrane fuel cells. Increasing Pt/C activity and stability is an urgent task and the optimization of their structure seems to be one of the possible solutions. In the present paper, Pt/C electrocatalysts containing small (2-2.6 nm) nanoparticles (NPs) of a similar size, uniformly distributed over the surface of a carbon support, were obtained by the original method of liquid-phase synthesis. A comparative study of the structural characteristics, catalytic activity in the oxygen electroreduction reaction (ORR), and durability of the synthesized catalysts, as well as their commercial analogs, was carried out. It was shown that the uniformity of the structural and morphological characteristics of Pt/C catalysts makes it possible to reduce the negative effect of the small size of NPs on their stability. As a result, the obtained catalysts were significantly superior to their commercial analogs regarding ORR activity, but not inferior to them in terms of stability.

Particle Size Distributions from Electron Microscopy Images: Avoiding Pitfalls

An important property of heterogeneous catalysts is the size distribution of the catalytically active phase. This is typically obtained form a long list of particles sizes (manually) compiled from electron micrographs. These raw data are then represented as histogram to approximate the underlying continuous distribution. Selecting the proper bin width, w, for the histogram is important as one has to balance resolution with statistical significance of the bin count in each bin. For most published particle size distributions, the selection criterion for w is not reported transparently. In this contribution, it is demonstrated how operator's bias can be avoided by using estimators for w that are based on the raw data only. First, synthetic data are analyzed to illustrate the importance of selecting a proper value for w. Then a survey of published data is presented which reveals that the values for the bin width w was chosen too large in many cases. By using statistically founded bin width estimators not only is operator's bias avoided but also hidden features in the distribution are sometimes revealed; in one case, a distinct bimodal distribution was missed in the original report. Finally, a work-flow is suggested which avoids operator's bias to generate particles size distributions from a list of experimentally determined particle sizes.

Particle Size Effect on Platinum Dissolution: Practical Considerations for Fuel Cells

The high costs of polymer membrane electrolyte fuel cells (PEMFCs) remain a roadblock for a competitive market with combustion engine vehicles. The PEMFC costs can be reduced by decreasing the size of Pt nanoparticles in the catalyst layer, thereby increasing the Pt dispersion and utilization. Furthermore, high-power performance loss due to O₂ transport resistance is alleviated by decreasing the particle size and increasing dispersion. However, firm conclusions on how Pt particle size impacts durability remain elusive due to synthetic difficulties in exclusively varying single parameters (e.g., particle size and loading). Therefore, here the particle size of Pt nanoparticles was varied from 2.0 to 2.8 and 3.7 nm while keeping the loading constant (30 wt %) on a Vulcan support using the two-step surfactant-free toolbox method. By studying the electrochemical dissolution in situ using online inductively coupled plasma mass spectrometry (online ICP-MS), mass-specific dissolution trends are revealed and are attributed to particle-size-dependent changes in electrochemically active surface area. Such degradation trends are critical for the start/stop of PEMFCs and currently require the implementation of potential control systems in consumer vehicles. Additionally, shifts in the onset of anodic dissolution and also oxidation to more negative potentials with decreasing particle size were observed. These results indicate a similar mechanism of anodic dissolution related to place-exchange when moving from extended polycrystalline Pt to nanoparticle scales. The negative shifts in the onset as the particle size decreases highlight a practical limitation for PEMFCs during load/idle conditions: without further material improvements, which inhibit Pt dissolution, reduction in costs and improvement in high-power performance via increased Pt utilization and dispersion will not be possible by decreasing particle sizes further.

Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study

To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt-Pt and Pt-Co coordination).

In situ synthesis of small Pt nanoparticles on chitin aerogel derived N doped ultra-thin carbon nanofibers for superior hydrogen evolution catalysis

Small and uniform Pt NPs were in situ synthesized on chitin aerogel derived N doped ultra-thin carbon nanofibers.